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Is there anything they can do in the USA to help prevent the tornadoes? Would planting forests on those flat landscapes help—Michael.

A spate of particularly severe tornadoes this year in the United States — and most recently New Zealand — has some people wondering whether people should grow more trees to slow the wind down.

Meteorologist Professor Michael Reeder from Monash University says it is understandable that people watching a tornado rip towards them across flat mid-western landscapes might think a forest would slow down the storm.

"But you'd be underestimating the tremendous power of a tornado. These storms would rip out the trees very easily. Trees aren't really a buffer for tornadoes."

Reeder says it's the not the details of landscape but the bigger picture, the underlying geography in that part of the US, that causes winds to gather such deadly ferocity.

"Tornadoes are formed from very severe thunderstorms, these are often called supercells and they're a very special type of very strong and long-lived storm. In the US these storms are especially common because of the position of the Rockies relative to the Gulf of Mexico. The warm moist air from the Gulf of Mexico lying underneath the cooler dryer air which in turn is lying over the Rockies, produces these very intense storms."

The land surface itself plays a role but it's just detail, he says, and if you were to plant more trees his guess is that the forested areas would be slightly warmer and slightly moister above the canopy. You may even provoke more storms, "but the bottom line is there really is no measurable effect."

The wet effect

However, some land surface phenomena can affect the weather, says Reeder.

"Perhaps where you normally get the biggest land surface induced effect on weather systems is when you change how wet the surface is, Reeder says.

"For example, when tropical cyclones make land fall they tend to die because there's no water. But if you've had one cyclone with a lot of rainfall and the land surface is saturated and a second tropical cyclone comes along, it can penetrate further inland and maintain its intensity because the land is now soggy and it still has water that it can draw on as fuel."

Bushfires can also have an impact, says Reeder, citing the 2003 Canberra fires as an example.

"When a forest burns it produces heat and moisture and it produces a lot of water as well, so for every kilogram of fuel burnt, about half a kilo of water is produced. In the case of Canberra, that additional heat and water produced large pyro-cumulus clouds — and the intense storms that resulted produced at least one genuine tornado."

But Canberra was an extreme and unusual example, Reeder adds.

In Reeder's opinion, the perception that weather events like tornadoes are getting more frequent and violent is not borne out by the evidence.

"There are plenty of reasons why we should be doing something about climate change" he says, "but storms are not one of them. There isn't any evidence and the models can't be used to say anything about that scale of weather events. At that scale all bets are off."